bugeye electrical question

My left rear taillight wasn't working so I checked the circuit and I have 11 
volts there.  I substituted the bulb socket with a different one and it 
worked.  Then after screwing it back its not working again.

Any suggestions where I should start checking now.  Is it possible to ruin 
the bulb holder if its shorting out somewhere.  There are no signs or smells 
of hot wires.

Thanks for any help.

Phil 

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